The background of the invention will be set forth in two parts.
1. Field of the Invention
This invention relates generally to transmission lines of radio frequency energy, and more particularly to coaxial cable type transmission lines.
2. Description of the Prior Art
Transmission lines of the coaxial type having long been known for their advantageous qualities such as the capability of generating no external field, and having no susceptibility to external fields from other sources.
A coaxial line is generally defined as a two conductor transmission line in which one conductor completely surrounds the other, the two being coaxial and separated by either a continuous solid dielectric material or by dielectric spacers with an inert gas as the principal insulating material. The latter type lines are generally expensive to manufacture and are of relatively large diameter in order to accomodate the distribution of the spacers uniformly throughout the length of the line. On the other hand, the solid dielectric type coaxial lines are relatively easily manufactured, even in small diameter sizes, but generally have a higher loss factor than those lines utilizing disk or bead dielectric spacers.
An important factor in evaluating both basic types of coaxial lines is that of their immunity from moisture contamination. The introduction of moisture between the two conductors of a coaxial line will greatly affect important propagation characteristics of the line. This problem is usually not a factor with gas-filled lines because the gas is under pressure and is necessarily hermetically sealed at its ends in order to maintain gas pressure.
However, solid dielectric coaxial lines are ordinarily provided with a braided metal wire outer conductor and are, therefore, susceptible to moisture contamination unless a special moisture-tight cover or jacket can be provided over the braid. Not only does the jacket add greatly to the cost and size of the cable, but they are generally only useful for a short time, and they are not hermetic. Eventually these covers will pass moisture. Also, these covers do not solve the inherent problem of field leakage involved in the use of a braided outer conductor in place of a solid outer conductor. In other words, braided outer conductor coaxial lines are generally more susceptible to problems involving external RF fields than coaxial lines having solid outer conductors.
There are also other applications where the smallness of the solid dielectric type lines are required, but requiring the complete and dependable freedom from moisture contamination found in gas-filled, segmented spacer type coax. One example of such a requirement would be in applications requiring hermetic RF feedthrough, such as used to bring a high speed electrical signal from a cooled photodetector inside a dewar to external circuitry. Presently, there are no relatively small diameter, solid dielectric hermetic coaxial cables available.